Petroleum refining operations in which crude oil is processed to produce gasoline, diesel fuel, lubricants and so forth, frequently produce residual oils. Residual oil, when processed in a delayed coker is heated in a furnace to a temperature sufficient to cause destructive distillation in which a substantial portion of the residual oil is converted, or “cracked” to usable hydrocarbon products and the remainder yields petroleum coke, a material composed mostly of carbon. Many oil refineries recover valuable products from the heavy residual hydrocarbons, which remain following delayed coking.
Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit, then pumping the heated heavy feed into a large steel vessel commonly known as a coke drum. The unvaporized portion of the heated heavy feed settles out in the coke drum, where the combined effect of retention time and temperature causes the formation of coke. Vapors from the top of the coke vessel are returned to the base of the fractionation unit for further processing into desired light hydrocarbon products. The operating conditions of delayed coking can be quite severe. Normal operating pressures in coke drums typically range from twenty-five to fifty pounds per square inch. Additionally, the heavy feed input temperature may vary between 800° F. and 1000° F.
The structural size and shape of the coke drum varies considerably from one installation to another. However, the typical coke drum is a large, upright, cylindrical, metal vessel commonly ninety to one-hundred feet in height, and twenty to thirty feet in diameter. Coke drums have a top head and a funnel shaped bottom portion fitted with a bottom head. Coke drums are usually present in pairs so that they can be operated alternately. Coke settles out and accumulates in a vessel until it is filled, at which time the heated feed is switched to the alternate empty coke drum. While one coke drum is being filled with heated residual oil, the other vessel is being cooled and purged of coke.
Coke removal, also known as decoking, begins with a quench step in which steam and then water are introduced into the coke filled vessel to complete the recovery of volatile, light hydrocarbons and to cool the mass of coke. After a coke drum has been filled, stripped and then quenched so that the coke is in a solid state and the temperature is reduced to a reasonable level, quench water is drained from the drum through piping to allow for safe unheading of the drum. The drum is then vented to atmospheric pressure when the bottom opening is unheaded, to permit removing coke. Once the unheading is complete, the coke in the drum is cut out of the drum by high pressure water jets.
Decoking is accomplished at most plants using a hydraulic system comprised of a drill stem and drill bit that direct high pressure water jets (2600–3600 p.s.i.) into the coke bed. A rotating combination drill bit, referred to as the cutting tool, is typically about eighteen inches in diameter with several nozzles, and is mounted on the lower end of a long hollow drill stem about six inches in diameter. The drill bit is lowered into the vessel, on the drill stem, through a flanged opening at the top of the vessel. A “bore hole” is drilled through the coke using the nozzles, which eject high pressure water at an angle approximately sixty degrees down from horizontal. This creates a pilot bore hole, about three to six feet in diameter, for the coke to fall through.
After the initial bore hole is complete, the drill bit is then mechanically switched to at least two horizontal nozzles in preparation for cutting the “cut” hole, which extends to the full drum diameter. In the cutting mode the nozzles shoot jets of water horizontally outwards, rotating slowly with the drill rod, and those jets cut the coke into pieces, which fall out the open bottom of the vessel, into a chute that directs the coke to a receiving area. In all employed systems the drill rod is then withdrawn out the flanged opening at the top of the vessel. Finally, the top and bottom of the vessel are closed by replacing the head units, flanges or other closure devices employed on the vessel unit. The vessel is then clean and ready for the next filling cycle with the heavy hydrocarbon feed.
In the typical coke-cutting system, after the boring hole is made, the drill stem must be removed from the coke drum and reset to the cutting mode. This takes time, is inconvenient and is potentially hazardous. In less typical systems the modes are automatically switched. Automatic switching within the coke drum oftentimes results in drill stem clogging, which still requires the drill stem to be removed for cleaning prior to completing the coke-cutting process. Often, in automatic switching systems, it is difficult to determine whether or not the drill stem is in cutting or boring mode, because the entire change takes place within the drum. Mistakes in identifying whether the high pressure water is cutting or boring lead to serious accidents. Thus, coke-cutting efficiency is compromised because the switching operator does not know whether or not the cutting process is complete or simply clogged.
Decoking is dangerous work. Serious incidents occur each year in connection with coke-cutting operations. OSHA Report entitled Hazards of Delayed Coker Unit (DCU) Operations, found at http://www.osha.gov/dts/shib/shib082903c.html (Aug. 29, 2003) which details several safety hazards associated with decoking. OSHA's report describes some of the most frequent and severe hazards. Id. The OSHA's report explains that if the hydro-cutting system is not shut off before the drill stem is raised out of the top drum opening, operators are exposed to the high-pressure water jet and serious injuries including dismemberment occur. Id. Additionally, the report adds that fugitive mists and vapors from the cutting and the quench water contain contaminants posing a health hazard. Id. Further, the water hose occasionally bursts while under high pressure, resulting in a whipping action that may seriously injure nearby workers. Alternatively, the wire rope supporting the drill stem and water hose could fail, allowing the drill stem, water hose, and wire rope to fall onto work areas. Id. Finally, gantry damage may occur, exposing workers to falling structural members and equipment. Id. Thus, operators are exposed to significant safety hazards from exposure to high pressure water jets, steam, hot water and fires because operators must be present, in close proximity to the vessel being decoked, to manually change the cutting head from the boring to cutting mode. Accordingly, the industry has concentrated most of their technological improvements in the field of coking to minimize the safety hazards.
Steps taken to control hazards inherent in coke-cutting systems consist of providing protective wear to the operators, requiring personnel training, maintaining equipment so that it is fail-proof, and allowing remote operation of certain steps of the decoking process (e.g., “deheading”). Despite efforts to reduce the hazards associated with decoking, there still exists a need for improved safety.